Production of slip cast calcia hollowware



United States Patent 3,116,350 PRODUCTIGN 0F SLlP CAST CALCIA HOLLOWWARE Stephen D. Stoddard, Donald E. Nuckolls, and Robert E. Cowan, Los Aiamos, N. Mex., assignors to the United States of America as represented by the United fitates Atomic Energy Commission No Drawing. Filed Feb. 14, 1961, Ser. No. 89,321 6 Claims. (Cl. 264--86) The present invention relates to slip casting of calcium oxide, or calcia, and more particularly to slip casting calcia as a monolith.

Such calcia structures are of general utility as containers for solid and liquid metals and alloys, particularly when the end sought is a minimum contamination of the contained metal by the container. They are thus of particular utility in containing nuclear reactor fuels such as plutonium and uranium, where contaminations of of a percent and less are significant. The containers may take the shape of crucibles, molds melt containers, combustion boats, reduction chamber liners, etc. A particular use is in the bomb process for the production of plutonium (or uranium) metal from its tetrafluoride by reduction with calcium and with the aid of an iodine booster, as disclosed in the US. patent to Baker, 2,890,111. Experimental work with calcia liners has demonstrated that the total impurities in the plutonium button resulting from the bomb process may be kept consistently below 200 parts per million, whereas the contamination using the magnesia crucibles of the prior art is many times such value.

Although the utility of calcia for such purposes has been known for some time, the prior art is almost completely silent about methods for obtaining well densified calcia shapes. For the most part calcia was tamped into place to form a lining over some other refractory material just prior to use of the container, a practice probably justified by the extreme hygroscopicity of ordinary powdered calcia. It has been found that as little as 0.5% water in a casting slip will cause the castings to crack. The British patent to Williams, 699,089, 1953, does teach the casting of calcia monoliths in conventional plaster molds by suspending finely ground calcia in ethyl or methyl alcohol, but in arriving at the present invention it was found that such alcohols are too hygroscopic. They absorb water from even low humidity air so fast that most of the castings crack before or during firing.

Accordingly, it is the primary object of the present invention to furnish methods of slip casting calcia in which so little moisture is absorbed in the casting slip as to eliminate cracking of the casting between the time of removing the casting from its mold and the completion of firing the shape.

Another object is to provide calcia slips for slip casting in which the suspending vehicle is non-aqueous, has a low vapor pressure or at least does not form a dry film on exposure to air, and low water solubility.

A further object is to furnish a method of producing calcia hollowware using such slips to produce green castings which do not crack during drying and firing and resuit in fired ware which is highly resistant to hydration in ordinary atmospheres.

The above and other objects are attained in the present invention primarily by the use of isobutyl acetate or a mixture of tertiary amyl alcohol and o-xylene as the suspending agents or vehicles. It is noted that from their very nature, both isobutyl acetate and a mixture of tertiary amyl alcohol and o-xylene possess the properties of being non-aqueous, relatively non-reactive toward both the plaster mold and the material being suspended, and prone toward not being hygroscopic. The preferred such sus- 3,llfi,350 Patented Dec. 31, 1963 "ice pending agent is isobutyl acetate, a liquid having a flash point of 18 C. wd hence requiring the precaution of use in areas where there are no open flames. Its solubility for water is so low that no diificulty from hydration of the calcia is encountered. While isobutyl acetate has a somewhat higher vapor pressure than water, it exhibits no objectionable tendency to form a dry film on the surface of the slip on exposure to air, the reason for desiring low vapor pressure. It is of moderate toxicity, but may be safely used in well ventilated areas.

It was found that mixtures of calcia or calcia and isobutyl acetate (and CaO with other organic vehicles as well) are quite viscous and require large amounts of the liquid to form a suitable slip. It is noted that the addition of a mixture of oleic acid and triethanolamine to the suspension, will, due to its very nature, greatly reduce its viscosity and permit the use of smaller percentages of liquid, and thus is greatly beneficial in reducing the drying and firing shrinkage of the cast ware. Small quantities of either triethanolamine or oleic acid have a like eiiect, but must be used in combination to attain the optimum eifect.

In preparing the slip, it was found until quite recently that all commercially available raw calcia contained considenable amounts of impurities which must be removed to optimize the purity of the final sintered article. Even more important, the individual particles of such raw calcia were quite low in density and thus require a preliminary calcination or, ideally, a fusing heat treatment to increase the density and lower the hygroscopicity of the particles prior to casting. Without such preliminary densification, the concomitant shrinkage would all take place during the drying and sintering of the green castings, with disastrous results.

Recently a fused and pure calcia has become available, making such preliminary purification and densification unnecessary. Even such newer material, however, requires considerable reduction in particle size in order that it may be suspended in the vehicles of the present invention without settling during the time required to cast a wall. To accomplish such reduction in size and at the same time forestall subsequent agglomeration, the calcia and suspending agent are ground in a porcelain ball mill until the average particle size of the calcia is of the order of 1.53.0 microns.

Such a uniformly fine suspension is not particularly desirable because it has been found that a calcia consis-ting entirely of an extremely fine particle size will result in a casting which undergoes greater drying and firing shrinkage than a coarser calcia or a mixture of fine and coarse calcia. To reduce such shrinkage, a coarser fraction of calcia may be added to the ball-milled slip, e. g., about 10% by weight of 2 lil mesh.

By contrast, it should be noted that calcia used to form a coating on other refractory oxides may be quite widely graded in particle size, e.g., to 270 mesh. Such calcia has been used, for instance, by stirring, for example, 100 gms. of calcia into a solution of 50 cc. ethanol and 25 cc. each of normal octyl alcohol and vythene (normal octyl alcohol and the vythene prevent water pickup) and pouring such slip into a slip cast, sintered magnesia crucible. Coating thicknesses of from 0.005 to 0.015 inch are formed in from 5 to 20 seconds, the coarser particles having no opportunity to settle out in such short times. Such slip cast coatings are sintered (fired) by the methods described in the examples below to produce articles used for the same purposes as the monolithic calcia articles of the present invention, in particular those used for the plutonium bomb reduction process.

Another casting slip of the present invention utilizes as the vehicle a combination of tertiary amyl alcohol and a xylene, preferably o-xylene, also known as dimethylbenzene, because it has the lowest vapor pressure and the highest boiling point of three xylene compounds.

either of these compounds is suitable in and of itself as a vehicle, tertiary amyl alcohol being extremely sensitive to such mold conditions as fingerprints or mold re lease agents on the plaster mold, with a resulting nonuniform wall thickness in the casting. o-Xylene, on the other hand, forms a very thick slip which is thixotropic. When used together, however, the undesirable characteristics of each compound disappear and the solution serves as :a highly satisfactor suspending agent for calcia. Ethyl cellulose is a useful addition, as it adds green strength to the ware, is insoluble in water and helps to control the casting rate. Another helpful addition is oleic acid, as it reduces viscosity and thus makes it possible to reduce the fraction of vehicle necessary to suspend a given quantity of calcia.

It has been found necessary to use a mold release in slip casting monolith calcia shapes according to the present invention. A suitable material is ball milled flake graphite, applied to the mold in xylene by pouring the suspension in and out of the mold as rapidly as possible.

To better enable those skilled in the art to understand the present invention, two examples are furnished below. These are merely illustrative, and it should be understood that no limitations are intended or implied thereby. The invention consists broadly in methods of obtaining useful calcia shapes by slip casting in novel vehicles. A wide range of compositions is possible as can be readily determined by those skilled in the art.

EXAMPLE 1 A one gallon ball mill charge included 2400 grams of 60 mesh calcined calcia, 648 cc. of isobutyl acetate, 4 cc. of oleic acid and 1 cc. of triethanolamine. The ball charge was 3185 grams of one inch diameter and 1015 grams of three-quarter inch high density alumina balls. The mill was rotated at 5 2 revolutions per minute for about 20 hours.

After milling, the viscosity of the slip was reduced by additions of oleic acid and triethanolamine, addition of the latter being about 1 drop per 100 grams of slip. The oleic acid was added wlule the slip was being stirred until no further decrease in viscosity could be observed. The exact amount of the addition varied somewhat from. batch to batch and varies with the type of calcia used, less being required for fused calcia.

This slip was cast into crucible molds of conventional plaster made of three parts water and four parts plaster, coated with a graphite mold release previously applied by pouring in a suspension of ball milled flake graphite in xylene and pouring out the suspension as rapidly as possible. The slip built up a one-eighth inch wall in 5 minutes, after which the mold was drained easily to leave behind a Well defined casting having sharp interior corners. The castings were separated from the molds and dried by heating at 80 C. until complete removal of the isobutyl acetate. These green castings had good strength and were easily trimmed.

Some of the green castings were then fired in electric or gas-air heated furnaces, on 60 mesh calcia grog and on setter plates of cast unfired calcia for 2-inch diameter and larger pieces and of fired magnesia for smaller castings. it is necessary that castings fired in a gas-air furnace be well muffled to avoid hydration when using a gasair furnace.

Crucible castings thus fired at 1200" C. had densities of 2.14 grams/cc, or 64% of theoretical, while others fired at 1725 C. for 20 minutes had densities of 2.72 grams/ cc., or 82% of theoretical.

Another group of the green castings was fired in a hydrogen furnace for 13 hours at 1850" C. The cnucibles thus fired were separated from a molybdenum setter plate by 60 mesh calcia grog. Their resulting densities were 3.14 grams per cc., or 94.6% of theoretical. They were translucent and completely gas tight. in addition, they i exhibited excellent hydration resistance, one sample exposed to an average 25% relative humidity for over 6 months showing no signs of deteriorating. In accelerated hydration tests at 53 C. and 96% relative humidity, only 15 of the calcia was converted to calcium hydroxide after a 75 hour exposure.

The above process has been used for making crucibles in sizes as large as 3 /slI1Ch outside diameter and 8%-inch height, with one-eighth inch wall.

Chemical analyses demonstrated that all products were quite pure, and that impurity levels in the slip cast crucibles decrease with increasing firing temperature and soaking time, as indicated in the following table.

Impurity Levels in CaO crucibles, p.p.m.

l,200 0., 1,725 C. 1,850 C., Element no soak, 20 min. 13 hr.soak,

in air soak, gas-air Hz 15 10 1 400 300 250 250 150 200 200 200 200 15 5 5 2 1 1 100 50 l 5 l 20 15 1 20 1 1 5O 30 30 EXAMPLE 2 The fiuffy as-received calcia was preliminarily punificd and the particles densified as follows: passing it through a 60 mesh screen to crush soft agglomerates, adding the particles to ethyl alcohol to form a thin slurry, passing the slurry through a magnetic separator to remove free or tramp iron, drying to remove the alcohol, briquetting, and firing the briquettes in a gas-air furnace in magnesia crucibles at 1760 C. for /2 hour. The briquettes were then broken to 4 mesh in a jaw crusher, and then to 60 mesh in a hammer mill, the final product having a high bulk density and a small surface area.

A one gallon ball mill charge was prepared from 2400 grams of such 60 mesh calcia, 791 milliliters of tertiary amyl alcohol, 339 milliliters of o-xylene, 6 grams of ethyl cellulose and 3.6 milliliters of oleic acid. The ball mill charge consisted of 3185 grams of one-inch diameter and 1015 grams of three-quarter inch high density alumina balls. The mill was rotated for 26 hours at 52 revolutions per minute.

After milling, the viscosity of the slip was reduced by adding more oleic acid, at the rate of 38.4 milliliters of oleic acid per 2400 grams of slip. To reduce drying and firing shrinkage, 200 grams of 200 mesh calcia, relatively coarse by comparison with the ball mill product, were added to 2400 grams of the latter just prior to casting and mixed into the slip with an electric stirrer.

The crucible molds used were of conventional plaster, containing three parts of water to four parts of plaster. Prior to casting they were coated with graphite prepared by ball milling flake graphite for 72 hours, leaching with hydrochloric acid to remove iron, and suspending the graphite in xylene. Coating was accomplished by quickly pouring the suspension in and out of the mold, leaving a thin, uniform coating on the inside of the mold with only a few seconds contact.

The slip is poured into the coated mold in open air without special precautions by Way of ventilation hoods. A one-eighth inch wall of calcia was deposited on the mold surfaces in eleven minutes, after which the balance of the slip was poured out easily, leaving a casting with sharp, well defined corners.

Some of the green cast crucibles were fired well mufiled in gas-air furnaces on +60 mesh (U-S. Standard Sieve Series) calcia grog, pieces greater than 2 inches in diameter on cast calcia setter plates and smaller pieces on fired magnesia setter plates. A firing temperature of 5 1725 C. for 20 minutes resulted in crucibles of 2.72 grams/cc, or 92% of theoretical. Such crucibles were translucent and nearly vitreous in appearance. Very small to quite large crucibles were thus cast, the largest being 3 /3 inches in 0.1)., 8% inches high, with a /a-inch wall.

What is claimed is:

l. A method for slip casting monolithic calcia hollC-WWQIB comprising the steps of ball milling a suspension of dense calcia grain in a vehicle selected from the class consisting of isobutyl acetate and a mixture of tertiary amyl alcohol and o-xylene and casting said suspension in a plaster mold coated with a mold release agent.

2. The method of claim 1 in which said vehicle is isohutyl acetate and said suspension includes a minor fraction of at least one viscosity controlling agent selected from the class consisting of oleic acid and triethanolamine.

3. The method of claim 1 in which said suspension vehicle includes a minor amount of triethanolaminc as a viscosity controlling agent.

4. The method of claim 1 in which said suspension vehicle includes in combination a minor amount of triethanolamine and oleic acid as viscosity controlling agents.

5. The method of claim 1 in which said vehicle is a mixture of tertiary amyl alcohol and o-Xylene and said suspension contains a minor fraction of an ethyl cellulose binder.

6. The method of claim 5 in which said suspension also contains a minor fraction of an oleic acid viscosity controlling agent.

References Cited in the file of this patent UNITED STATES PATENTS 2,320,009 Ralsto n May 25, 1943 2,546,477 Sewell Mar. 27, 1951 2,876,122 Whittemore Mar. 3, 1959 FOREIGN PATENTS 699,089 Great Britain Get. 28, 1953 OTHER REFERENCES Kingery, W. D.: Ceramic Fabrication Processes, John Wiley and Son, Inc., N.Y., 1958, T.P. 807 16/62 Library of Congress No. 586*077. (Chapter 5, pp. 4551 relied upon.) 

1. A METHOD FOR SLIP CASTING MONOLITHIC CALCIA HOLLOWWARE COMPRISING THE STEPS OF BALL MILLING A SUSPENSION OF DENSE CALCIA GRAIN IN A VEHICLE SELECTED FROM THE CLASS CONSISTING OF ISOBUTYL ACETATE AND A MIXTURE OF TERTIARY AMYL ALCOHOL AND O-XYLENE AND CASTING SAID SUSPENSION IN A PLASTER MOLD COATED WITH A MOLD RELEASE AGENT. 